The present invention relates to the processing of a C3 to C6 hydrocarbon cut from a cracking process, such as steam or fluid catalytic cracking, primarily for conversion of C4 olefins to propylene via metathesis.
In typical olefin plants, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene and C2 acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds ranging in carbon number from C3 to C6. This mixture is separated into different carbon numbers typically by fractionation.
The C3 cut, primarily propylene, is removed as product and is ultimately used for the production of polypropylene or for chemical synthesis such as propylene oxide, cumene, or acrylonitrile. The methyl acetylene and propadiene (MAPD) impurities must be removed either by fractionation or hydrogenation. Hydrogenation is preferred since some of these highly unsaturated C3 compounds end up as propylene thereby increasing the yield.
The C4 cut consisting of C4 acetylenes, butadiene, iso- and normal butenes, and iso- and normal butane can be processed in many ways. A typical steam cracker C4 cut contains the following components in weight %:
C4 acetylenestracebutadiene33%1-butene15%2-butene9%isobutylene30%iso- & normal butane13%Typically, the butadiene and C4 acetylenes are removed first. This can be accomplished by either hydrogenation or extraction. If extraction is employed, the remaining 1-butene and 2-butene remain essentially in the same ratio as that of the initial feedstock. If hydrogenation is employed, the initial product from butadiene hydrogenation is 1-butene. Subsequently, hydroisomerization occurs within the same reaction system changing the 1-butene to 2-butene. The extent of this reaction depends upon catalyst and reaction conditions within the hydrogenation system. However, it is common practice to limit the extent of hydroisomerization in order to avoid “over hydrogenation” and the production of butanes from butenes. This would represent a loss of butene feedstock for subsequent operations downstream. The butenes remaining in the mixture consist of normal olefins (1-butene, 2-butene) and iso-olefins (isobutylene). The balance of the mixture consists of both iso- and normal butanes from the original feed plus what was produced in the hydrogenation steps and any small quantity of unconverted or unrecovered butadiene.
The butenes have many uses. One such use is for the production of propylene via metathesis. Another is for the production of ethylene and hexene via metathesis. Conventional metathesis involves the reaction of normal butenes (both 1-butene and 2-butene) with ethylene. These reactions occur in the presence of a catalyst which consists of a group VIA or VII A metal oxide either supported or unsupported. Typical catalysts for metathesis are tungsten oxide supported on silica or rhenium oxide supported on alumina. Isobutylene (isobutene) is typically removed from the feedstock prior to the metathesis reaction step. The reaction of isobutylene with ethylene is non-productive and reaction with itself and/or other C4's is limited in the presence of excess ethylene. Non-productive reactions essentially occupy catalyst sites but produce no product. If allowed to remain in the feed to the metathesis unit, the concentration of this non-reactive species would build up creating capacity limitations. The reaction of 1-butene with ethylene is also non-productive. However, it is common to employ a double bond isomerization catalyst within the metathesis reactor to shift 1-butene to 2-butene and allow for continued reaction. Typical double bond isomerization catalysts include basic metal oxides (Group IIA) either supported or unsupported. Magnesium oxide or calcium oxide are examples of such double bond isomerization catalysts that are physically admixed with the metathesis catalyst. No equivalent co-catalyst exists for the skeletal isomerization of isobutylene to normal butene. In the case of the conventional metathesis system employing both a metathesis catalyst and a co-mixed double bond isomerization catalyst, the butadiene must be removed to a level of less than 500 ppm to avoid rapid fouling of the double bond isomerization catalyst. The metathesis catalyst itself can tolerate butadiene levels up to 10,000 ppm.
It is common to employ an isobutylene removal step prior to metathesis. Options include reacting it with methanol to produce methyl tertiary butyl ether (MTBE) or separating the isobutylene from the butenes by fractionation. In U.S. Pat. No. 6,358,482, the inventors teach the removal of isobutylene from the C4 mixture prior to metathesis. This scheme is further reflected in U.S. Pat. Nos. 6,075,173 and 5,898,091. In other prior art, in U.S. Pat. No. 6,580,009, Schwab et al teach a process for the production of propylene and hexene from a limited ethylene fraction. For molar ratios of ethylene to butenes (expressed as n-butenes) from 0.05 to 0.60, the inventors utilize a raffinate II stream as the C4 feedstock. A raffinate II stream is by definition a stream following isobutylene removal. Further, in claim 6 of the published application, they teach the steps of: (1) removal of butadiene; (2) removal of isobutylene by various means; (3) removal of the oxygenate impurities; (4) metathesis of the resultant raffinate II stream with 0.05 to 0.6 molar ratio of ethylene; (5) separation of products to form a lighter fraction of C2 and C3 stream and a heavier fraction C4 to C6 stream; (6) separating the lighter fraction into C2 and C3 product streams; (7) separating the heavier stream into a C4 stream, an intermediate C5 stream and a heavier C6+ stream; and (8) recycle of the unreacted C4 and at least partially the C5 normal olefins. This process specifically removes the isobutylene prior to metathesis.
In U.S. Pat. No. 6,271,430, Scwab et al disclose a two-step process for the production of propylene. The first step consists of reacting 1-butene and 2-butene in a raffinate II stream in an autometathesis reaction to form propylene and 2-pentene. The products are then separated in the second step. The third step reacts specifically the 2-pentene with ethylene to form propylene and 1-butene. This process utilizes the isobutylene free raffinate II stream. The pentenes recycled and reacted with ethylene are normal pentenes (2-pentene).
It is known that isobutylene removal from the C4 stream can also be accomplished by employing a combined catalytic distillation hydroisomerization deisobutyleneizer system (CD DIB system). This system will both remove the isobutylene and recover n-butenes at high efficiency by isomerizing the 1-butene to 2-butene with known isomerization catalysts and thus increase the volatility difference. This technology combines conventional fractionation for isobutylene removal with hydroisomerization within a catalytic distillation tower. In U.S. Pat. No. 5,087,780 to Arganbright, 2-butene is hydroisomerized to 1-butene as the fractionation occurs. This allows greater than equilibrium amounts of 1-butene to be formed as the mixture is separated. Similarly, 1-butene can be hydroisomerized to 2-butene in a catalytic distillation tower. In separating a C4 stream containing isobutylene, 1-butene, and 2-butene (plus paraffins), it is difficult to separate isobutylene from 1-butene since their boiling points are very close. By employing simultaneous hydroisomerization of the 1-butene to 2-butene with fractionation of isobutylene (CD DIB), isobutylene can be separated from the normal butenes at high efficiency.
The metathesis reaction described above is equimolar, i.e., one mol of ethylene reacts with 1 mol of 2-butene to produce 2 mols of propylene. However, commercially, in many cases, the quantity of ethylene available is limited with respect to the quantity of butenes available. In addition, the ethylene is an expensive feedstock and it is desired to limit the quantities of ethylene used. As the ratio of ethylene to butenes is decreased, there is a greater tendency for the butenes to react with themselves which reduces the overall selectivity to propylene.
The metathesis catalysts and the double bond isomerization catalysts are quite sensitive to poisons. Poisons include water, CO2, oxygenates (such as MTBE), sulfur compounds, nitrogen compounds, and heavy metals. It is common practice to employ guard beds upstream of the metathesis reaction system to insure the removal of these poisons. It does not matter if these guard beds are directly before the metathesis reaction system or further upstream as long as the poisons are removed and no new poisons are subsequently introduced.
Metathesis reactions are very sensitive to the location of the olefin double bond and the stereo-structure of the individual molecules. A pair of olefins adsorb on the surface and exchange double bond positions with the carbon groups on either sides of the double bonds. Metathesis reactions can be classified as productive, half productive or non-productive. As described above, non-productive reactions result in essentially no reaction taking place. When the double bonds shift with metathesis reaction, the new molecules are the same as the originally adsorbed molecules thus no productive reaction occurs. This is typical for reactions between symmetric olefins or reactions between ethylene and alpha olefins. If fully productive reactions occur, new products are generated no matter which orientation the molecules occupy the sites. The reaction between ethylene and 2-butene to form two propylene molecules is a fully productive reaction. Half productive reactions are sterically inhibited. If the pair of olefins adsorb in one orientation, when the double bonds shift new products are formed. Alternately if they adsorb in a different steric configuration, when the bonds shift, the identical olefins are formed and thus no new products are formed. The various metathesis reactions proceed at different rates (a fully productive reaction is usually faster than a half productive reaction) and with different weight selectivities to propylene. Table A summarizes the reactions between ethylene and various butenes and the reactions between the butenes themselves.
The reactions listed represent the base reaction with ethylene (reaction 1, 4 and 5) as well as the reactions between the various C4 olefins. It is especially important to make a distinction between the selectivity to propylene from total C4 olefins (including isobutylene) and the selectivity to propylene from the normal C4 olefins involved in the reaction. The reaction of isobutylene with 2-butene (reaction 6) produces propylene and a branched C5 molecule. For this reaction, propylene is produced at 37.5 weight % selectivity from total C4's (similar to reaction 2) but at a 75 weight % selectivity from normal C4's (2-butene). For the purposes of definitions, conventional metathesis is defined as the reaction of the C4 olefin stream with ethylene. However, the C4 stream can also react without the presence of ethylene as a feedstock. This reaction is called auto or self metathesis. In this case, reactions 2,3 6, and 7 are the only possible reactions and will occur at rates dependent upon the feedstock composition.
TABLE AWt %Wt %selectivityselectivity(C3H6 fromC3H6 fromReactionRatetotal C4's)(n-C4's)12-butene + ethylene → 2Fast100100propylene (conventionalmetathesis) (fully productive)21-butene + 2-butene →Fast37.537.5propylene + 2-pentene(fully productive)31-butene + 1-butene →Slow00ethylene + 3-hexene(half productive)4isobutylene + ethylene →No Rknno reaction51-butene + ethylene →No Rknno reaction6isobutylene + 2-butene →Fast37.575propylene + 2-Me 2-butene(fully productive)7isobutylene + 1-butene →Slow00ethylene + 2-me 2-pentene(half productive)It is common practice in conventional metathesis to maximize reaction 1 to produce propylene. This will maximize the selectivity to propylene. As such, excess ethylene is used to reduce the extent of the reactions of butenes with themselves (reactions 2, 3, 6, and 7). The theoretical ratio is 1/1 molar or 0.5 weight ratio of ethylene to n-butenes but it is common in conventional metathesis to employ significantly greater ratios, typically, 1.3 or larger molar ratio to minimize reactions 2, 3, 6 and 7. Under conditions of excess ethylene, and due to the fact that both isobutylene and 1-butene do not react with ethylene (see reactions 4 and 5), two process sequences are employed. First, the isobutylene is removed prior to metathesis. If isobutylene is not removed, it will build up as the n-butenes are recycled to achieve high yield. Second, 1-butene is isomerized to 2-butene by including a double bond isomerization catalyst such as magnesium oxide admixed with the metathesis catalyst. Note that this catalyst will not cause skeletal isomerization (isobutylene to normal butylenes) but only shift the double bond from the 1 position to the 2 position. Thus by operating with excess ethylene, eliminating isobutylene from the metathesis feed prior to reaction, and employing a double bond isomerization catalyst, reaction 1 is maximized.
When there is limited or no fresh ethylene (or excess butylenes for the ethylene available), there are currently two options available for propylene production. In these cases, the current technology will first remove the isobutylene and then process the normal butenes with whatever ethylene is available. The entire n-butenes only mixture is subjected to metathesis with the available ethylene. Ultimately, if there is no fresh ethylene available, the C4's react with themselves (auto metathesis). Under low ethylene conditions, reactions 2, 3, 6 and 7 will occur, all leading to a lower propylene selectivity (37.5% or lower versus 100% for reaction 1). The lower selectivity results in lower propylene production. Note that reactions 6 and 7 will be minimized as a result of the removal of isobutylene (to low levels but not necessarily zero). Alternately, the molar flows of ethylene and butenes can be matched by limiting the flow of butenes to produce conditions where there is a high selectivity of the normal butenes to propylene via reaction 1. By limiting the flow of n-butenes to match ethylene, the production of propylene is limited by the reduced butenes flow.
Pentenes and some hexenes are formed to some extent in the conventional metathesis case with low ethylene. The volume of these components will depend upon the ethylene/n-butenes ratio with a lower ratio producing more C5 and C6 components. In the conventional prior art case where isobutylene is removed before any metathesis, these C5 and C6 olefins are normal olefins since no skeletal isomerization occurs. It is possible to recycle these olefins back to the metathesis step where for example the reaction with ethylene and 2-pentene will occur yielding propylene and 1-butene. The 1-butene is recovered and recycled. Note however, with limited ethylene, reaction 1 can occur only to the limit of the ethylene availability. Ultimately these non-selective byproducts, pentenes and hexenes, must be purged from the system. While the presence of these olefins impacts the required size of the equipment, they do not represent losses of potential propylene production.